unassertive (adj.)

• abject
• accepting
• acquiescent
• agreeable
• backward
• bashful
• complaisant
• compliant
• consenting
• diffident
• modest
• nonresistant
• obedient
• passive
• retiring
• self-effacing
• servile
• shy
• submissive
• subservient
• supine
• timid
• unassured
• uncomplaining
• unresistant
• unresisting